Creating a Python web application with Flask

With this tutorial, you will develop a simple web application from scratch: a mini blog where users can create, view, edit, and delete posts. To do this, you will create a Yandex Compute Cloud VM in Yandex Cloud and run a web server on it.

The web app is written in Python and uses SQLite to work with data. To develop the frontend, we will use Bootstrap.

We opted for Flask as the base framework for our application, since it is one of the most intuitive and straightforward Python web frameworks.

The Flask framework allows you to create web applications from a single source code file; it does not require following any specific directory structure or writing complex template code before you start using it. Flask also has out-of-the-box support for the Werkzeug toolkit and the Jinja templating engine to create dynamic HTML pages.

The source code for the application is available in the flask_blog.zip file.

Application workflow:

1. HTTP requests are sent from the user's browser to a Linux Ubuntu VM which has a Flask web server installed and running.
2. The web server forwards a request to the web application, whose router invokes the appropriate handler function based on the request URL.
3. The handler function executes a query to the SQLite database to retrieve or write the required data.
4. The function sends the data retrieved from the database to the appropriate Jinja HTML template, which returns the final HTML code of the page.
5. This HTML code is forwarded to the web server, which in turn serves this code to the user's browser.

To create a web application with Flask:

1. Prepare your cloud.
2. Create and set up a virtual machine.
3. Create and run the application.
4. Create and configure the HTML templates.
5. Configure the database.
6. Display blog posts.
7. Add actions with posts.
8. Summarize the results.

If you no longer need the web application, delete the resources it uses.

Getting startedGetting started

In Yandex Cloud, you only pay for the resources you consume and the time of their actual use. A billing account is required to identify the user who pays for such resources.

The cost of support for a web application includes:

• Fee for using a public IP address (see Yandex Virtual Private Cloud pricing).
• Fee for VM computing resources and disks (see Yandex Compute Cloud pricing).

Sign up for Yandex Cloud and create a billing account:

1. Go to the management console and log in to Yandex Cloud or create an account if you do not have one yet.
2. On the Yandex Cloud Billing page, make sure you have a billing account linked and it has the ACTIVE or TRIAL_ACTIVE status. If you do not have a billing account, create one.

If you have an active billing account, you can go to the cloud page to create or select a folder for your infrastructure to operate in.

Learn more about clouds and folders.

Create a cloud network and subnetCreate a cloud network and subnet

In Yandex Cloud, resources are connected to each other and the internet using cloud networks, in which resources receive public IP addresses and private IP address ranges, or subnets.

To create a virtual network and subnet for your web server:

Create a security groupCreate a security group

Security groups are the main tool for managing network access in Yandex Cloud and help restrict unauthorized traffic to cloud resources at the cloud network level.

Create a security group that allows inbound TCP traffic on ports 5000 and 22 as well as any outbound traffic. Port 22 is used to connect to the VM via SSH, while 5000 is the default port to run a Flask web server.

yc vpc security-group create \
  --name webserver-sg \
  --rule "description=Flask,direction=ingress,port=5000,protocol=tcp,v4-cidrs=[0.0.0.0/0]" \
  --rule "description=ssh,direction=ingress,port=22,protocol=tcp,v4-cidrs=[0.0.0.0/0]" \
  --rule "description=any,direction=egress,port=any,protocol=any,v4-cidrs=[0.0.0.0/0]" \
  --network-name webserver-network

id: enpv1c0q7j01********
folder_id: b1gt6g8ht345********
created_at: "2024-03-23T18:54:05Z"
name: webserver-sg
network_id: enp9mji1m7b3********
status: ACTIVE
rules:
  - id: enpmbsk2hjfd********
    description: Flask
    direction: INGRESS
    ports:
      from_port: "5000"
      to_port: "5000"
    protocol_name: TCP
    protocol_number: "6"
    cidr_blocks:
      v4_cidr_blocks:
        - 0.0.0.0/0
  - id: enpna5id9265********
    description: ssh
    direction: INGRESS
    ports:
      from_port: "22"
      to_port: "22"
    protocol_name: TCP
    protocol_number: "6"
    cidr_blocks:
      v4_cidr_blocks:
        - 0.0.0.0/0
  - id: enpen3vf7rui********
    description: any
    direction: EGRESS
    protocol_name: ANY
    protocol_number: "-1"
    cidr_blocks:
      v4_cidr_blocks:
        - 0.0.0.0/0

Create and set up a virtual machineCreate and set up a virtual machine

A virtual machine is similar to a server in the cloud infrastructure. In Yandex Cloud, you can create VMs with different hardware characteristics in terms of performance, RAM, and disk space, as well as running different operating systems.

This web application will be deployed on an Ubuntu 22.04 LTS Linux VM.

1. Create a virtual machine:

   Before you start, prepare a key pair (public and private keys) to access your VM over SSH.

   Management console

   CLI

   API

   1. In the management console, select the folder to create your VM.

   2. In the list of services, select Compute Cloud.

   3. In the left-hand panel, select Virtual machines.

   4. Click Create virtual machine.

   5. Under Boot disk image, select the Ubuntu 22.04 LTS image.

   6. Under Location, select the ru-central1-b availability zone.

   7. Under Network settings:

      • In the Subnet field, select the webserver-subnet-ru-central1-b subnet you created earlier.
      • In the Public IP field, select Auto.
      • In the Security groups field, select the webserver-sg security group you created earlier.

   8. Under Access, specify the information required to access the VM:

      • Enter the username in the Login field: yc-user.

      • In the SSH key field, select the SSH key saved in your organization user profile.

        If there are no saved SSH keys in your profile, or you want to add a new key:

        • Click Add key.
        • Enter a name for the SSH key.
        • Upload or paste the contents of the public key file. You need to create a key pair for the SSH connection to a VM yourself.
        • Click Add.

        The SSH key will be added to your organization user profile.

        If users cannot add SSH keys to their profiles in the organization, the added public SSH key will only be saved to the user profile of the VM being created.

   9. Under General information, specify the VM name: sftp-server.

   10. Click Create VM.

   Run the command specifying the security group ID you saved at the previous step:

   yc compute instance create \
     --name mywebserver \
     --zone ru-central1-b \
     --network-interface subnet-name=webserver-subnet-ru-central1-b,nat-ip-version=ipv4,security-group-ids=<security_group_ID> \
     --create-boot-disk image-folder-id=standard-images,image-id=fd8ne6e3etbrr2ve9nlc \
     --ssh-key <public_SSH_key_file>
   

   Where --ssh-key is the path to the file with the public SSH key, e.g., ~/.ssh/id_ed25519.pub.

   Result:

   done (32s)
   id: epdv79cu67np********
   folder_id: b1gt6g8ht345********
   created_at: "2024-03-23T19:17:09Z"
   name: mywebserver
   zone_id: ru-central1-b
   platform_id: standard-v2
   resources:
     memory: "2147483648"
     cores: "2"
     core_fraction: "100"
   status: RUNNING
   metadata_options:
     gce_http_endpoint: ENABLED
     aws_v1_http_endpoint: ENABLED
     gce_http_token: ENABLED
     aws_v1_http_token: DISABLED
   boot_disk:
     mode: READ_WRITE
     device_name: epdg0926k12t********
     auto_delete: true
     disk_id: epdg0926k12t********
   network_interfaces:
     - index: "0"
       mac_address: d0:0d:1f:3a:59:e3
       subnet_id: e2l3qffk0h6t********
       primary_v4_address:
         address: 192.168.1.14
         one_to_one_nat:
           address: 62.84.***.***
           ip_version: IPV4
       security_group_ids:
         - enpv1c0q7j01********
   serial_port_settings:
     ssh_authorization: INSTANCE_METADATA
   gpu_settings: {}
   fqdn: epdv79cu67np********.auto.internal
   scheduling_policy: {}
   network_settings:
     type: STANDARD
   placement_policy: {}
   

   For more information about the yc compute instance create command, see the CLI reference.

   To create a VM, use the create REST API method for the Instance resource or the InstanceService/Create gRPC API call.

   This will create the mywebserver VM in your folder. To connect to the VM over SSH, use the yc-user username and the VM’s public IP address. If you plan to use the created web server over a long period of time, make this VM's public IP address static.

2. Create and activate the virtual environment.

   A virtual environment provides an isolated space for Python projects on the server. All required dependencies, e.g., executables, libraries, and other files, are copied to a selected folder, and the application uses them instead of those installed on the system. This ensures the stability of the development environment and keeps the base system clean.

   To create a virtual environment, use the Python 3 venv library module:

   1. Connect to the mywebserver VM.

   2. In the current user's directory, create a project subdirectory named flask_blog where the application will reside and navigate into it:

      mkdir flask_blog && cd flask_blog
      

   3. Install the venv virtualization module:

      sudo apt install python3.10-venv
      

   4. Create the env virtual environment:

      python3 -m venv env
      

   5. Activate the virtual environment:

      source env/bin/activate
      

      After you activate the virtual environment, a prefix with the name of the environment will appear in the command line:

      (env) yc-user@ubuntu:~/flask_blog$
      

      Note

      To effectively track and manage the project development process, you can use a version control system. In this case, add the env directory to the .gitignore file to only track files related to the project.

      To deactivate the virtual environment, run this command:

      deactivate
      

3. Install Flask:

   pip install flask
   

   Result:

   Successfully installed Jinja2-3.1.3 MarkupSafe-2.1.5 Werkzeug-3.0.1 blinker-1.7.0 click-8.1.7 flask-3.0.2 itsdangerous-2.1.2
   

Create and run the applicationCreate and run the application

Create a simple web application inside a Python file and run it to start the server.

1. In the flask_blog project directory, create and open the app.py file:

   nano app.py
   

   This file allows you to understand how the application will handle HTTP requests.

2. Add the following code to the file:

   from flask import Flask
   
   app = Flask(__name__)
   
   
   @app.route('/')
   def hello():
       return 'Hello, World!'
   

   In this code:

   • A Flask object is imported from the flask package.
   • An instance of the Flask application named app is created. The special __name__ variable contains the name of the current Python module and indicates its location to the instance. This is necessary because Flask creates several routes within the application.
   • The @app.route('/') decorator transforms the Python function into a Flask view function. The view function converts the return value into an HTTP response that can be processed by an HTTP client, such as a web browser. The '/' value in @app.route()​​ indicates that this function will respond to web requests for the / URL, which is the primary URL of the web application.
   • The hello() function is created that returns the Hello, World! string as a response.

   Save and close the app.py file.

3. Set the Flask environment variables:

   export FLASK_APP=app && export FLASK_DEBUG=true
   

   Where:

   • FLASK_APP=app indicates the location of the app, i.e., the app.py file.
   • FLASK_DEBUG=true indicates the application should run in the development mode.

4. Run the application:

   flask run \
     --host=0.0.0.0
   

   Where the --host=0.0.0.0 parameter starts the application server on all IP addresses of the VM. If you do not specify this parameter, you will not be able to access the application using the VM's public IP address.

   Result:

    * Serving Flask app 'app'
    * Debug mode: on
   WARNING: This is a development server. Do not use it in a production deployment. Use a production WSGI server instead.
    * Running on all addresses (0.0.0.0)
    * Running on http://127.0.0.1:5000
    * Running on http://192.168.1.14:5000
   Press CTRL+C to quit
    * Restarting with stat
    * Debugger is active!
    * Debugger PIN: 884-371-***
   

   You will need the following information:

   • Name of the running application: app.
   • Debug mode: on: Indicates that the Flask debugger is active. This feature is useful in development because it provides detailed error messages when problems occur, making it easier to resolve them.
   • The application is running on all addresses.

   You can change the web server port to any other value by setting this optional parameter: --port=<port_number>. For example, the flask run --host=0.0.0.0 --port=5001 command will start the Flask web server on port 5001.

5. In you browser address bar, enter http://<public_IP_address_of_VM>:5000/​.

   The browser will display the Hello, World! string. This means that your application is running properly and can be accessed from the internet.

Keep the web server running in the current window and open a new terminal window. You will use this new window to finish setting up your application. This way, you can test your changes without having to stop and restart the web server.

When you open a new terminal window:

1. Connect to the VM.

2. Go to the flask_blog project directory, activate the virtual environment, and set environment variables:

   cd flask_blog && source env/bin/activate && export FLASK_APP=app && export FLASK_DEBUG=true
   

Create and configure the HTML templatesCreate and configure the HTML templates

At this point, the application can only display a plain text message in the browser. To display the data and controls that the user needs, you need to add HTML templates to your application.

To use templates in the app, you can use the render_template() helper function enabled by the Jinja template engine. You will use HTML templates to create the application pages: the main page listing blog posts and the pages for creating, viewing, and editing a post.

Create the main page HTML templateCreate the main page HTML template

1. Open the app.py file:

   nano app.py
   

2. Replace the file contents with this code:

   from flask import Flask, render_template
   
   app = Flask(__name__)
   
   @app.route('/')
   def index():
       return render_template('index.html')
   

   In this code:

   • The render_template() helper function is imported to render HTML templates.
   • The hello() function is replaced with the index() function that returns the result of invoking the render_template() helper function with the index.html​​ argument. The function argument points to a template file in the template directory to use for rendering.

   Note

   You will create the template directory and the index.html file later. Running the application at this stage will return an error.

   Save and close the app.py file.

3. In the flask_blog project directory, create a subdirectory named templates:

   mkdir templates
   

4. Create and open the index.html template file in the templates directory:

   nano templates/index.html
   

5. Add the following code to the index.html file:

   <!DOCTYPE html>
   <html lang="en">
     <head>
       <meta charset="UTF-8">
       <title>FlaskBlog</title>
     </head>
     <body>
       <h1>Welcome to FlaskBlog</h1>
     </body>
   </html>
   

   This HTML code describes a simple page with the FlaskBlog header and Welcome to FlaskBlog as the first level header.

   Save and close the index.html file.

6. Refresh the main page of your application, http://<public_IP_address_of_VM>:5000/.

   Result:

   Welcome to FlaskBlog.
   

7. In addition to the templates directory, Flask web applications typically have the static directory that stores static files, such as CSS styles, JavaScript scripts, or images used by the application.

   In the flask_blog project directory, create a subdirectory named static:

   mkdir static
   

8. In the static directory you just created, create the css subdirectory:

   mkdir static/css
   

   Note

   Subdirectories are used to organize static files in dedicated folders. For example, the js directory usually stores JavaScript files, the images (or img) directory stores images, etc.

9. In the static/css project directory, create and open the style.css file:

   nano static/css/style.css
   

10. Add the following code to the style.css file:

    h1 {
        border: 2px #eee solid;
        color: #fc3d17;
        text-align: center;
        padding: 10px;
    }
    

    This CSS code changes the formatting of the text wrapped by the <h1> HTML tags:

    • border adds a border.
    • color changes the text color.
    • text-align sets text alignment.
    • padding adds a padding between the element's border and text.

    Save and close the style.css file.

11. Open the index.html template:

    nano templates/index.html
    

12. Add a link to style.css inside the <head> section:

    ...
      <head>
        <meta charset="UTF-8">
        <link rel="stylesheet" href="{{ url_for('static', filename= 'css/style.css') }}">
        <title>FlaskBlog</title>
      </head>
    ...
    

    The link uses the url_for() helper function​​​ to generate the URL for the file location. The first argument indicates that the link references a static file; the second argument specifies the path to the file in the static file subdirectory.

    Save and close the index.html file.

13. Refresh the main page of your application, http://<public_IP_address_of_VM>:5000/.

    You will see that the Welcome to FlaskBlog​​​ text changed its color to red and is now centered inside the frame.

The CSS language allows you to create a consistent and unique design for your web application. If you are not familiar with CSS, use the Bootstrap toolkit that provides simple and easy-to-use UI components. In this tutorial, you will use Bootstrap.

Configure other HTML templatesConfigure other HTML templates

In other HTML templates, you will need to reuse most of the HTML code from the index.html template. To avoid unnecessary duplicate code, use a base template file as a parent for all other HTML templates in the project. See more in the Jinja documentation.

1. Create and open the base.html base template in the templates directory:

   nano templates/base.html
   

2. Add the following code to the file:

   <!doctype html>
   <html lang="en">
     <head>
       <!-- Required meta tags -->
       <meta charset="utf-8">
       <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
   
       <!-- Bootstrap CSS -->
       <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.2.3/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-rbsA2VBKQhggwzxH7pPCaAqO46MgnOM80zW1RWuH61DGLwZJEdK2Kadq2F9CUG65" crossorigin="anonymous">
   
       <title>{% block title %} {% endblock %}</title>
     </head>
     <body>
       <nav class="navbar navbar-expand-lg bg-light">
         <div class="container-fluid">
           <a class="navbar-brand" href="{{ url_for('index')}}">FlaskBlog</a>
           <button class="navbar-toggler" type="button" data-bs-toggle="collapse" data-bs-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
             <span class="navbar-toggler-icon"></span>
           </button>
           <div class="collapse navbar-collapse" id="navbarSupportedContent">
             <ul class="navbar-nav me-auto mb-2 mb-lg-0">
               <li class="nav-item">
                 <a class="nav-link active" href="#">About</a>
               </li>
             </ul>
           </div>
         </div>
       </nav>
       <div class="container">
         {% block content %} {% endblock %}
       </div>
   
       <!-- Optional JavaScript -->
       <script src="https://cdn.jsdelivr.net/npm/bootstrap@5.2.3/dist/js/bootstrap.bundle.min.js" integrity="sha384-kenU1KFdBIe4zVF0s0G1M5b4hcpxyD9F7jL+jjXkk+Q2h455rYXK/7HAuoJl+0I4" crossorigin="anonymous"></script>
     </body>
   </html>
   

   This file contains HTML code as well as additional code required by Bootstrap. The <meta> tags contain browser information, the <link> tag binds Bootstrap CSS files, and the <script> tag refererces a JavaScript script that enables additional Bootstrap features. For more information, see the Bootstrap documentation.

   The following code elements are specific to the Jinja template engine:

   • {% block title %} {% endblock %}: Placeholder block for a header. You will use it in other templates to give a custom header for each page in your application without rewriting the <head> section.
   • {{ url_for('index')}}: Calls a function that returns the URL for the index() view function. This call is different from url_for() used previously to bind the CSS file. In this case, you need to provide only one argument, the name of the view function linked to its associated route and not a file.
   • {% block content %} {% endblock %}: Block that will contain content specific to each child template.

   Save and close the base.html file.

3. Configure your HTML templates to inherit code from the base template. Open the index.html template:

   nano templates/index.html
   

4. Replace the file contents with this code:

   {% extends 'base.html' %}
   
   {% block title %} Welcome to FlaskBlog {% endblock %}
   
   {% block content %}
     <h1> Welcome to FlaskBlog </h1>
   {% endblock %}
   

   In this code:

   • {% extends 'base.html' %}: Indicates inheritance from the base.html template.
   • {% block title %} ... {% endblock %}: Specifies a header that will replace the variable in the corresponding block of the base.html parent template.
   • {% block content %} ... {% endblock %}: Specifies the content that will replace the variable in the corresponding block of the base.html parent template.

   Since Welcome to FlaskBlog is used as both the page name and the header that appears below the navigation bar, you can make this code even more concise and avoid repeating the same text twice:

   {% extends 'base.html' %}
   
   {% block content %}
     <h1>{% block title %} Welcome to FlaskBlog {% endblock %}</h1>
   {% endblock %}
   

   Save and close the index.html file.

5. Refresh the main page of your application, http://<public_IP_address_of_VM>:5000/.

   Your page will have a navigation bar and a styled header.

Configure the databaseConfigure the database

Create a database to store blog posts and populate it with sample entries.

You will use SQLite to work with the web application data because the sqlite3 module, which you will need to work with the database, comes bundled with Python.

Prepare the schema.sql file containing SQL commands to create a database consisting of a table named posts with multiple columns.

1. In the flask_blog project directory, create and open the schema.sql file:

   nano schema.sql
   

2. Add the following code to the file:

   DROP TABLE IF EXISTS posts;
   
   CREATE TABLE posts (
       id INTEGER PRIMARY KEY AUTOINCREMENT,
       created TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
       title TEXT NOT NULL,
       content TEXT NOT NULL
   );
   

   In this code:

   • The DROP TABLE IF EXISTS posts; command deletes the posts table if it already exists. We recommend performing this step each time you create a new table to prevent possible errors.
   • The CREATE TABLE posts command creates the posts table with the following columns:
     • id: Integer that is the PRIMARY KEY of the table. AUTOINCREMENT means that its value will be a unique number generated automatically when inserting a new record (blog post) into the table.
     • created: New record timestamp. NOT NULL indicates that the value in the column cannot be empty. DEFAULT CURRENT_TIMESTAMP​​​ sets the default value as the time when a new record is added to the table. This means that you do not need to manually specify the time when a post is created, it will be added automatically.
     • title: Post title.
     • content: Main text of the post.

   Save and close the schema.sql file.

   Note

   When using these SQL commands, any previous contents of the posts table in the database.db file will be deleted. Do not save anything important to the database via the web application until you have completed this tutorial.

3. In the flask_blog project directory, create and open the init_db.py file:

   nano init_db.py
   

   This file will create the SQLite database file for your web application.

4. Add the following code to the file:

   import sqlite3
   
   connection = sqlite3.connect('database.db')
   
   
   with open('schema.sql') as f:
       connection.executescript(f.read())
   
   cur = connection.cursor()
   
   cur.execute("INSERT INTO posts (title, content) VALUES (?, ?)",
               ('First Post', 'Content for the first post')
               )
   
   cur.execute("INSERT INTO posts (title, content) VALUES (?, ?)",
               ('Second Post', 'Content for the second post')
               )
   
   connection.commit()
   connection.close()
   

   In this code:

   • The sqlite3 module is imported.
   • The connection to the database.db file is opened; this file is created once you run the Python file.
   • The open() function opens the previously created schema.sql SQL schema.
   • The executescript() method formalizes the contents of the schema by running multiple SQL statements at once and creating the posts table.
   • The cursor() method opens the cursor for sending queries to the database.
   • The execute() method executes two INSERT SQL statements, adding two sample posts to the posts table.
   • The changes are committed and the connection is closed.

   Save and close the init_db.py file.

5. Run the file using python:

   python init_db.py
   

6. Make sure the database.db file is in the flask_blog project directory:

   ls -l
   

   Result:

   total 40
   -rw-rw-r-- 1 yc-user yc-user   105 Mar 23 19:45 app.py
   -rw-r--r-- 1 yc-user yc-user 12288 Mar 24 12:27 database.db
   drwxrwxr-x 5 yc-user yc-user  4096 Mar 23 19:30 env
   -rw-rw-r-- 1 yc-user yc-user   472 Mar 24 12:27 init_db.py
   drwxrwxr-x 2 yc-user yc-user  4096 Mar 23 19:45 __pycache__
   -rw-rw-r-- 1 yc-user yc-user   204 Mar 24 12:26 schema.sql
   drwxrwxr-x 2 yc-user yc-user  4096 Mar 24 12:29 static
   drwxrwxr-x 2 yc-user yc-user  4096 Mar 24 12:30 templates
   

Display blog postsDisplay blog posts

On the main page of your application, customize the display of the list of all blog posts and create a page to view individual posts by their ID.

Display the list of all blog postsDisplay the list of all blog posts

Update the index() view function and the index.html template to display a list of all posts stored in the database.

1. Open the app.py file:

   nano app.py
   

2. Replace the file contents with this code:

   import sqlite3
   from flask import Flask, render_template
   
   def get_db_connection():
       conn = sqlite3.connect('database.db')
       conn.row_factory = sqlite3.Row
       return conn
   
   app = Flask(__name__)
   
   @app.route('/')
   def index():
       conn = get_db_connection()
       posts = conn.execute('SELECT * FROM posts').fetchall()
       conn.close()
       return render_template('index.html', posts=posts)
   

   This code does the following as well:

   • Imports the sqlite3 module.
   • Creates the get_db_connection() function which:
     • Creates the conn object for connecting to the database.db database.
     • Assigns database data to the row_factory attribute of the conn object.
     • Returns the conn object which will be used to access the database.
   • The updated index() function:
     • Assigns the database connection object returned by get_db_connection() to conn.
     • Uses the fetchall() method to assign the posts object the result of the SQL query for selecting all the records from the posts table.
     • Uses the close() method to close the connection to the database.
     • Returns the HTML code of the application main page, formed from the index.html template, to which the posts object containing all records of the table with posts is provided as an argument.

   Save and close the app.py file.

3. Open the index.html template of the main page:

   nano templates/index.html
   

4. Replace the file contents with this code:

   {% extends 'base.html' %}
   
   {% block content %}
     <h1>{% block title %} Welcome to FlaskBlog {% endblock %}</h1>
     {% for post in posts %}
       <a href="#">
         <h2>{{ post['title'] }}</h2>
       </a>
       <span class="badge text-bg-primary">{{ post['created'] }}</span>
       <hr>
     {% endfor %}
   {% endblock %}
   

   This code adds:

   • {% for post in posts %} ... {% endfor %}: Jinja loop which displays, in order, each element contained in the posts object provided by the index() function in app.py. Inside this loop, for each post, the post title with a link is displayed in the <h2>​​ HTML header. Later on, these links will be used to open a view page for each post.
   • {{ post['title'] }}: Post title from the post variable, i.e. the element of the posts object processed in the current iteration of the for loop.
   • Similarly, the post['created'] post creation date is displayed with the badge CSS class applied to it.

   Save and close the index.html file.

5. Refresh the main page of your application, http://<public_IP_address_of_VM>:5000/.

   You will see a list of the two post titles you added to the database when creating it.

Display individual blog postsDisplay individual blog posts

Create a new Flask route with a view function that will process a new HTML template to display an individual blog post by its ID. The application will use the new route to display the post at http://<public_IP_address_of_VM>:5000/ID with the appropriate ID, if such an ID exists in the database.

1. Open the app.py file:

   nano app.py
   

2. Replace the file contents with this code:

   import sqlite3
   from flask import Flask, render_template
   from werkzeug.exceptions import abort
   
   def get_db_connection():
       conn = sqlite3.connect('database.db')
       conn.row_factory = sqlite3.Row
       return conn
   
   def get_post(post_id):
       conn = get_db_connection()
       post = conn.execute('SELECT * FROM posts WHERE id = ?',
                           (post_id,)).fetchone()
       conn.close()
       if post is None:
           abort(404)
       return post
   
   app = Flask(__name__)
   
   @app.route('/')
   def index():
       conn = get_db_connection()
       posts = conn.execute('SELECT * FROM posts').fetchall()
       conn.close()
       return render_template('index.html', posts=posts)
   
   @app.route('/<int:post_id>')
   def post(post_id):
       post = get_post(post_id)
       return render_template('post.html', post=post)
   

   This code does the following as well:

   • Imports the abort() function from the Werkzeug library. It allows you to generate a Flask response with the 404 Not Found​​​ message if the blog post with the ID specified in the URL does not exist.
   • Creates the get_post() function that takes the post_id argument with the blog post ID to retrieve. This function:
     • Assigns the database connection object returned by get_db_connection() to conn.
     • Uses the fetchone() method to assign the post object the result of the SQL query which retrieves the string with the blog post and its ID mapping to the post_id value from the database.
     • Uses the close() method to close the connection to the database.
     • Uses the abort() function to respond with the 404 error code if the post with the ID provided to post_id is not found, and then terminates.
     • If the post with the ID provided to post_id is found, the function returns the post object containing the database table row for this post.
   • Creates the post(post_id) view function. This function:
     • Uses the decorator to add the <int:post_id> variable rule specifying that the URL after / is a positive integer (marked by the int converter) used by the view function.
     • With the get_post(post_id) function, assigns the post object the contents of the database table row mapped to the blog post with the specified ID.
     • Returns the HTML code of the post view page formed from the post.html template, to which the post object containing the post contents is provided as an argument.

   Save and close the app.py file.

3. Create and open the post.html template file in the templates directory:

   nano templates/post.html
   

4. Add the following code to the file:

   {% extends 'base.html' %}
   
   {% block content %}
       <h2>{% block title %} {{ post['title'] }} {% endblock %}</h2>
       <p>{{ post['content'] }}</p>
       <span class="badge text-bg-primary">{{ post['created'] }}</span>
   {% endblock %}
   

   This code is similar to that of the index.html template. The difference is that it does not use the for loop here since the template should display a single post with its contents specified in post['content'].

   Save and close the post.html file.

5. To view the first two posts you created together with the database, open these URLs: http://<public_IP_address_of_VM>:5000/1 and http://<public_IP_address_of_VM>:5000/2.

   If you try to open http://<public_IP_address_of_VM>:5000/3, you will see an error page stating that the requested blog post is not found.

6. Add links from the post titles that open the post view pages to the main page of your application.

   Open the index.html template:

   nano templates/index.html
   

7. Replace the # value of the href attribute with {{ url_for('post', post_id=post['id']) }} so that the for loop is as follows:

   ...
     {% for post in posts %}
       <a href="{{ url_for('post', post_id=post['id']) }}">
         <h2>{{ post['title'] }}</h2>
       </a>
       <span class="badge text-bg-primary">{{ post['created'] }}</span>
       <hr>
     {% endfor %}
   ...
   

   The url_for() function takes the following arguments:

   • 'post': post() view function.
   • post_id: Variable with the post['id'] value used by the view function.

   The post['id'] function returns a valid URL for each post title based on the post ID.

   Save and close the index.html file.

8. Go to the main page of your application, http://<public_IP_address_of_VM>:5000/, and make sure that the post titles work as links that open the posts.

Add actions with postsAdd actions with posts

Enable users to create, edit, and delete posts in your application.

Creating a new postCreating a new post

Create a page where the user can add a new post with a title.

1. Open the app.py file:

   nano app.py
   

2. Replace the file contents with this code:

   import sqlite3
   from flask import Flask, render_template, request, url_for, flash, redirect
   from werkzeug.exceptions import abort
   
   def get_db_connection():
       conn = sqlite3.connect('database.db')
       conn.row_factory = sqlite3.Row
       return conn
   
   def get_post(post_id):
       conn = get_db_connection()
       post = conn.execute('SELECT * FROM posts WHERE id = ?',
                           (post_id,)).fetchone()
       conn.close()
       if post is None:
           abort(404)
       return post
   
   app = Flask(__name__)
   app.config['SECRET_KEY'] = 'your secret key'
   
   @app.route('/')
   def index():
       conn = get_db_connection()
       posts = conn.execute('SELECT * FROM posts').fetchall()
       conn.close()
       return render_template('index.html', posts=posts)
   
   @app.route('/<int:post_id>')
   def post(post_id):
       post = get_post(post_id)
       return render_template('post.html', post=post)
   
   @app.route('/create', methods=('GET', 'POST'))
   def create():
       return render_template('create.html')
   

   This code does the following as well:

   • Imports:
     • requestglobal object to access the data provided through the HTML form.
     • url_for() function to generate URLs.
     • flash() function to display a message when processing a request.
     • redirect() function to redirect the client to a different page.
   • Through the app.config object, adds the SECRET_KEY configuration required for the flash() function to store pop-up messages in the client's browser session. The secret key is a long string of random characters. It is used to create secure sessions and allows Flask to remember information between requests, for example, go from the post creation page to the main application page. A user can access the information stored in a session, but cannot edit it without the secret key. This is why you should never give anyone access to your secret key. For more information, see the Flask documentation.
   • Uses the create() view function to return the HTML code of the post creation page generated from the create.html template. The decorator creates the /create route that accepts GET and POST requests. By default, only GET requests are accepted. To ensure that the route also accepts POST requests, which are used by the browser when sending form data, you need to provide a tuple of valid request types in the methods argument.

   Save and close the app.py file.

3. Create and open the create.html template file in the templates directory:

   nano templates/create.html
   

4. Add the following code to the file:

   {% extends 'base.html' %}
   
   {% block content %}
   <h1>{% block title %} Create a New Post {% endblock %}</h1>
   
   <form method="post">
     <div class="mb-3">
       <label for="title" class="col-sm-2 col-form-label">Title</label>
       <input type="text" name="title"
              placeholder="Post title" class="form-control"
              value="{{ request.form['title'] }}">
     </div>
   
     <div class="mb-3">
       <label for="content" class="col-sm-2 col-form-label">Content</label>
       <textarea name="content" placeholder="Post content"
                 class="form-control" rows="3">{{ request.form['content'] }}</textarea>
     </div>
     <div class="mb-3">
       <button type="submit" class="btn btn-primary">Submit</button>
     </div>
   </form>
   {% endblock %}
   

   Basically, this is plain HTML code that will display fields for the title and body of the future post, as well as a button for submitting the form.

   The post title value is stored in the {{ request.form['title'] }} object, and the content value is stored in the {{ request.form['content'] }} object. This way the data you enter will not get lost if something goes wrong. For example, if you add the body text of a post but forget a title, you will see a message saying that the title is a required field. However, you will not lose the body text, as it will be saved in the request global object that you have access to in your templates.

   Save and close the create.html file.

5. Open the http://<public_IP_address_of_VM>:5000/create URL and check that you can see the Create a New Post page with input fields for the post title and contents, and the submit form button.

   The form on this page sends a POST request to the create() view function. However, the function currently lacks the code to handle the POST request, so the input data will not be saved to the database after the form is submitted. You can fix this by updating the create() view function.

6. Open the app.py file:

   nano app.py
   

7. Replace the contents of the create() view function with the code below:

   ...
   @app.route('/create', methods=('GET', 'POST'))
   def create():
       if request.method == 'POST':
           title = request.form['title']
           content = request.form['content']
   
           if not title:
               flash('Title is required!')
           else:
               conn = get_db_connection()
               conn.execute('INSERT INTO posts (title, content) VALUES (?, ?)',
                            (title, content))
               conn.commit()
               conn.close()
               return redirect(url_for('index'))
   
       return render_template('create.html')
   

   This function code:

   • Employs the if request.method == 'POST' condition to make sure the code is executed only for POST requests.
   • Retrieves the new post title and contents submitted in the form fields from the request.form global object which contains the form data.
   • If no post title is specified in the form, the if not title condition will be executed and the user will see a message informing them to fill in the title field.
   • If the title is specified:
     • The get_db_connection() function opens a connection to the database and a new post record is added to the posts table, containing the title and post text received through the form.
     • The changes are committed and the connection is closed.
     • The redirect() function redirects the user to your application's main page.

   Save and close the app.py file.

8. Open the http://<public_IP_address_of_VM>:5000/create URL, enter the title and contents of the post, and submit the form. After submitting the form, you will be redirected to your application's main page where you will see a list of all posts, including the new one.

9. In the base.html template, add a pop-up message display and a link to the new post form to the navigation bar. Open the base.html template:

   nano templates/base.html
   

10. Before the About​​ link inside the <nav> tag, add <li>:

    ...
        <nav class="navbar navbar-expand-lg bg-light">
          <div class="container-fluid">
            <a class="navbar-brand" href="{{ url_for('index')}}">FlaskBlog</a>
            <button class="navbar-toggler" type="button" data-bs-toggle="collapse" data-bs-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
              <span class="navbar-toggler-icon"></span>
            </button>
            <div class="collapse navbar-collapse" id="navbarSupportedContent">
              <ul class="navbar-nav me-auto mb-2 mb-lg-0">
                <li class="nav-item">
                  <a class="nav-link active" href="{{url_for('create')}}">New Post</a>
                </li>
                <li class="nav-item">
                  <a class="nav-link active" href="#">About</a>
                </li>
              </ul>
            </div>
          </div>
        </nav>
    ...
    

11. Under <div class="container">, just before the content section, add the for loop to display pop-up messages under the navigation bar:

    ...
        <div class="container">
          {% for message in get_flashed_messages() %}
            <div class="alert alert-danger">{{ message }}</div>
          {% endfor %}
          {% block content %} {% endblock %}
        </div>
    ...
    

    Pop-up messages are available in the special get_flashed_messages() Flask function.

    Save and close the base.html file.

    The New Post control linked to the /create route will appear on the navigation bar.

12. Go to the application's main page, http://<public_IP_address_of_VM>:5000, and make sure you can create a new post using the New Post link on the navigation bar.

Editing an existing postEditing an existing post

Create a page where you can edit an existing post.

1. Open the app.py file:

   nano app.py
   

2. Add the edit() view function to the end of the file:

   ...
   @app.route('/<int:id>/edit', methods=('GET', 'POST'))
   def edit(id):
       post = get_post(id)
   
       if request.method == 'POST':
           title = request.form['title']
           content = request.form['content']
   
           if not title:
               flash('Title is required!')
           else:
               conn = get_db_connection()
               conn.execute('UPDATE posts SET title = ?, content = ?'
                           ' WHERE id = ?',
                           (title, content, id))
               conn.commit()
               conn.close()
               return redirect(url_for('index'))
   
       return render_template('edit.html', post=post)
   

   Editing an existing post is similar to creating one, so this view function is almost identical to the create() view function. The post to edit is specified in the URL, and the post ID is provided to the edit() function in the id argument. The same value is also provided to the get_post() function to retrieve the current contents of the post from the database. The application receives the updated data in a POST request, which is processed in the if request.method == 'POST' condition.

   As with creating a new post, the function extracts the data from the request.form object into separate variables, and then checks that the user has filled in the post title. If the title is missing, the function displays a pop-up message saying that this is a required field. In other cases, the function opens a connection to the database and updates the fields in the row with the required post in the posts table. The ID of the post in the database matches the ID specified in the URL.

   In the case of a GET request, the function returns HTML code generated from the edit.html template with the post title and text values from the post object.

   Save and close the app.py file.

3. Create and open the edit.html template file in the templates directory:

   nano templates/edit.html
   

4. Add the following code to the file:

   {% extends 'base.html' %}
   
   {% block content %}
   <h1>{% block title %} Edit "{{ post['title'] }}" {% endblock %}</h1>
   
   <form method="post">
     <div class="mb-3">
       <label for="title" class="col-sm-2 col-form-label">Title</label>
       <input type="text" name="title" placeholder="Post title"
              class="form-control"
              value="{{ request.form['title'] or post['title'] }}">
     </div>
   
     <div class="mb-3">
       <label for="content" class="col-sm-2 col-form-label">Content</label>
       <textarea name="content" placeholder="Post content"
                 class="form-control" rows="3">{{ request.form['content'] or post['content'] }}</textarea>
     </div>
     <div class="mb-3">
       <button type="submit" class="btn btn-primary">Submit</button>
     </div>
   </form>
   <hr>
   {% endblock %}
   

   The code is similar to the create.html template code, except for the {{ request.form['title'] or post['title'] }} and {{ request.form['content'] or post['content'] }} expressions. These expressions display the data stored in the request, or if the request does not exist, the data from the post variable provided to the template with the current database values.

   Save and close the edit.html file.

5. Open http://<public_IP_address_of_VM>:5000/1/edit to edit the first post. You will see the Edit First Post page with the post details. Edit the post, submit the form, and check that the post is updated.

6. On the main application page, add a link to the edit page for each post in the list. Open the index.html template:

   nano templates/index.html
   

7. Add a link to edit the post after its creation date:

   ...
     {% for post in posts %}
       <a href="{{ url_for('post', post_id=post['id']) }}">
         <h2>{{ post['title'] }}</h2>
       </a>
       <span class="badge text-bg-primary">{{ post['created'] }}</span>
       <a href="{{ url_for('edit', id=post['id']) }}">
         <span class="badge text-bg-warning">Edit</span>
       </a>
       <hr>
     {% endfor %}
   ...
   

   This adds the Edit link to the post titles on the main page. This link points to the edit() view function and provides the post ID value from post['id'] to it.

   Save and close the index.html file.

8. Go to the application's main page, http://<public_IP_address_of_VM>:5000, and check that it has Edit links to edit posts.

Deleting a postDeleting a post

1. Open the app.py file:

   nano app.py
   

2. Add the delete() view function to the end of the file:

   ...
   @app.route('/<int:id>/delete', methods=('POST',))
   def delete(id):
       post = get_post(id)
       conn = get_db_connection()
       conn.execute('DELETE FROM posts WHERE id = ?', (id,))
       conn.commit()
       conn.close()
       flash('"{}" was successfully deleted!'.format(post['title']))
       return redirect(url_for('index'))
   

   This function only accepts POST requests. This means that opening http://<public_IP_address_of_VM>:5000/ID/delete in a browser will return an error, as web browsers use GET requests by default. You can only send a POST request via this route by using a form that provides the ID of the post you want to delete.

   After receiving a POST request, the function opens a connection to the database and executes the DELETE FROM SQL command, and then the changes are committed, and the connection is closed. The application redirects the user to the main page and displays a message that the post was successfully deleted.

   The Delete button will be added to the post edit page, so you do not need a separate template for this feature.

   Save and close the app.py file.

3. Open the edit.html template:

   nano templates/edit.html
   

4. Add the <form> tag right after the <hr> tag and before the {% endblock %} line:

   ...
   <hr>
   <form action="{{ url_for('delete', id=post['id']) }}" method="POST">
     <input type="submit" value="Delete Post"
            class="btn btn-danger btn-sm"
            onclick="return confirm('Are you sure you want to delete this post?')">
   </form>
   {% endblock %}
   

   This code uses the confirm() method to ask the user to confirm their action before sending a request.

   Save and close the edit.html file.

5. Go to the application's main page, http://<public_IP_address_of_VM>:5000, and check that the post edit page now has the Delete Post button.

Your application is now ready. If you did everything correctly, the final source code of your project should match the code on this page.

Summarize the resultsSummarize the results

In this tutorial, you created a simple web application using the Flask framework, SQLite DBMS, Werkzeug toolkit, Jinja template engine, and Bootstrap. The result is a fully functional blog that allows you to create, read, edit, and delete posts using URL parameters and HTML forms.

You can take this project further with a wide range of community-created Flask extensions, e.g.:

• Flask-Login: Allows you to manage user sessions, i.e., handles login/logout tasks and user session management.
• Flask-SQLAlchemy: Streamlines the use of Flask by supporting SQLAlchemy. It is a Python SQL toolkit and Object Relational Mapper for working with databases.
• Flask-Mail: Helps you send emails from Flask applications.

Delete the resources you createdDelete the resources you created

To stop paying for the deployed web server, delete the VM.

If you reserved a static public IP address for your web server, delete it.